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A History and Overview of the Trucking Industry Part 3

Truck size is also determined by dimensions of overall length, width, height and wheelbase. The wheelbase of a truck or tractor is the space between the centerline of the front axle and the centerline of a tandem assembly. Each state has its own rules pertaining to vehicle size and weight. Most federal highways allow the maximum gross vehicle weight to be 80,000 lbs for five axles. One single rear axle is typically allowed to carry up to 20,000 lbs, and tandem axles can carry 34,000 lbs. The front steer axles on a tractor are usually limited to 12,000 lbs. Some straight trucks, such as dump trucks, can have up to 20,000 lb ratings on steer axles. Most federal highways allow the maximum vehicle height to be 13’6” and width 102 inches. Overall length of rigs is determined by each individual state.

Some states, rather than restrict overall vehicle length, will restrict trailer lengths or trailer wheelbase lengths. Some states will allow larger gross vehicle weights, provided that the axle weight restrictions are legal. Certain state highways may have tighter or looser restrictions. A permit is required for special cargo that must violate restrictions. Truck companies and drivers need to be aware of the allowances and restrictions within the state in which they are operating.

There are many different types of semi-trailers. Combination rigs can be composed of different types of wheel and axle arrangements:

Rocky Mountain double: Is a rig with double trailers and a truck with three axles. The 40- 45-foot semi trailer has tandem axles. There is also a single-axle dolly and a 27- to 28-foot single-axle semi trailer. The rig has a total of seven axles and is not legal in some states.

Standard double: Is a single-axle tractor pulling a 28-foot semi trailer and a 28-foot trailer and is not legal in some states.

Turnpike double: Is a tandem axle tractor pulling a 48-foot semi trailer and a 48-foot trailer and are not legal in some states.

18-wheeler: Is the most familiar combination rig. The tractor has 10 wheels and the semi trailer has eight wheels; there are five axles on an 18-wheeler.

In order to comply with the various size and weight restrictions, yet still haul the maximum load possible, many truck combinations have been developed such as:

  • A straight truck with two or more axles on it
  • A tractor + a semi trailer with three or more axles on it
  • A truck + full trailer with four or more axles
  • A tractor + semi trailer + full trailer having five axles with two 26 to 29 foot trailers, also known as a double
  • A tractor + semi trailer + full trailer + full trailer having seven axles with three 26 to 29 foot trailers, also known as a triple
  • Seven axles with one 45 to 48 foot semi trailer and one 26 to 28 foot full trailer, or a Rocky Mountain double
  • Nine axles with two 45 to 48 foot trailers, or an eastern turnpike double
  • Special tractor semi trailer semi trailer eight axles, or an Ontario special
  • Eleven axles is called a Michigan special

Any long combination vehicle, or LCV, must still conform to all federal, state, and local laws which limit the total size and weight of the articulated vehicle. Many are limited to specific states or highways within states. Doubles are common on all interstates, but the use of triples is confined mainly to northwestern states.

Design Specifications for Trucks

Trucks designed today are assembled using components manufactured by hundreds of different manufacturers. To spec a truck is to select amongst the hundreds of options and features to create one suited to the buyer’s preference. The final list of all of the components that were used to go into the manufacturing of a specific truck or trailer is called a line sheet, spec sheet or build sheet. This sheet can be requested from the manufacturer or authorized dealer.

Of the many truck manufacturers on the market, the most popular and largest ones are Navistar, Peterbilt, Freightliner, Kenworth, Mack and Volvo/White. Mack is the only manufacturer that produces most of the components used in its vehicles. Volvo manufactures its own engines.

After a buyer selects a manufacturer and truck style, he/she is provided with an extensive list of components built by OEM’s, or original equipment manufacturers. The buyer can choose which type of engine, transmission, number of axles, suspension, brakes and instrumentation among other things.

Many factors play into what type of components should be selected for a new truck. If it is intended for over the road use, a sleeper berth will need to be installed for the driver. Will it be driving in adverse weather conditions, or mountainous regions often and need additional braking capacity? Are chrome wheels desired, or painted wheels?

If the vehicle will mainly be used off-road in construction sites it will need bigger transmission and rear axle ratios, possibly even more gears for it to start on sloped or low traction ground surfaces. However, an over-the-road truck needs to be geared at higher speeds for its application. Even over-the-road trucks will have different applications.

When considering the specs for a truck, trucks set to run in mountainous regions such as Colorado, need to be designed with gradeability in mind. It would need to have the gear ratios and horsepower to handle the differing grades. A truck only driven in the Midwestern states probably would not need such considerations.

What the truck will be doing day-to-day is a crucial factor in spec’ing a vehicle. The types of loads, speeds and terrains expected for it should determine its drive train components. The engine, transmission, axles and tire sizes must be configured to provide the desired vehicle performance.

Cost is a huge concern in the trucking industry. Not only is initial cost of the vehicle important, but so are the expenses of maintenance, fuel and vehicle load capacity as well as when the truck is out of service (downtime). For instance, minute changes in fuel economy can have a big effect on profitability in an over-the-road truck that racks up several hundred thousand miles a year. To ultimately reduce costs, the truck must be designed to run at the desired speed while using minimal amounts of fuel and still maintaining the necessary performance capability. Gear fast/run slow is a term used in the trucking industry when describing spec’ing a truck that will be geared to run at a high road speed while still maintaining a low engine speed. A lower engine speed means lower fuel consumption.

The application of the vehicle also determines what type of suspension it has. Air suspension typically results in a smoother ride for the driver and cargo. Drivers appreciate the better ride that air suspension offers. Conversely, a local haul tractor may fare better with leaf spring suspension due to cost.

The amount of duties a driver has in addition to driving may influence what type of transmission is installed. An automatic transmission may work the best for a delivery vehicle or garbage hauler because of the numerous stops and loading and unloading by the driver. However, a standard transmission would be more cost effective for an over-the-road operation where the truck’s miles are driven at highway speeds.

By choosing the right components, it can make a truck more lightweight, which means it takes more cargo for it before reaching its maximum weight restriction. If a truck’s metal is mainly composed of lightweight aluminum, it will cost more initially because aluminum is more expensive than steel. However, the up-front costs will pay for themselves in the long run due to the increased load capacity the truck will be able to carry in its lifetime. Aluminum is usually used for cabs, wheels, air tanks, fuel tanks and sometimes frames, front axles, transmissions and axle housings. Another way to save weight is to use centrifuge brake drums and lightweight fifth wheels.

Materials need to be transported efficiently and economically and trucks are meant to accomplish that task. They are available with the size, weight-carrying capability and configurations needed to haul any load. If such a truck is not available, most likely it can be built. When a truck is spec’d according to desired performance and works within the legal restrictions, an efficient and safe truck can be built to accomplish pretty much any task.


A modern diesel engine is composed of a multitude of parts that have been designed and engineered to work together to produce enough power to transport tons of pounds across the country with maximum efficiency at the cheapest cost. Misuse or the malfunction of one single component will affect the entire unit. This will result in anything from reduced efficiency to total failure. In order to insure optimal and consistent operation, engines must be properly used and maintained.

To help extend engine life and get the most out of an engine, maintenance such as oil changes and filter replacements need to be regularly scheduled. Periodic inspections allow for early detection and diagnosis of problems as well as keeping the engine in optimal operating conditions. In turn, this will help with power and fuel consumption from the engine. It also helps insure drivers and passengers are kept safe and reach their destination without delay.

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